Just in case Four Seasons luxury residence owners who have sued to block construction of a nearby condominium tower that will also house the Mexican Museum didn't know what they were up against, they do now.

More than 200 people, including members of the consular corps and top city officials like Mayor Ed Lee and District Attorney George Gascón, had a little party in their backyard on Monday to praise the planned 480-foot tower. The crowd on Jessie Square Plaza across Mission Street from Yerba Buena Gardens was studded with city leaders past and present, including former Mayor Willie Brown, now a Chronicle columnist, who lives nearby in the St. Regis.

The event was ostensibly to celebrate the Mexican Museum's new home on Mexican Independence Day, but it also appeared to be a show of force as Four Seasons' residents pursue their lawsuit and explore putting a measure on the ballot that would prevent new buildings from shadowing city parks.

The ballot measure would expand and strengthen Proposition K, which voters approved in 1984, and could impact virtually all big developments in San Francisco as the city looks to get denser in certain areas, including a new crop of skyscrapers south of Market Street.

"I do want to signal my support for this" development, Lee said after officials unfurled a red banner reading "Future Home of the Mexican Museum" on the yellow plywood surrounding the planned construction site. "I think it's not premature to announce that the entire board and the mayor are together on this, and the way we've done it. I think it's appropriate that we unite our community around how important this institution is for us."

Neighbors opposing the tower say they welcome the museum, they just want it in a shorter building.

Neighbors file suit

"We're very, very supportive of the Mexican Museum," said Matthew Schoenberg, president of the group Friends of Yerba Buena, a resident of the Four Seasons and one of the people who recently filed the lawsuit in Sacramento Superior Court.

"We think that project can be accomplished, just with a slightly shorter building that would respect Proposition K, and with some work on the pedestrian and traffic safety issues," Schoenberg said.

The measure's proponents were undeterred by Monday's gathering of political firepower as drumming from a traditional Aztec dance troupe and the strains of an all-woman mariachi band reverberated from the square.

"The museum should be here. It's the proper place for it," Schoenberg said. "But so far we have not been successful in getting people to compromise a little bit on the height of the building."

His group is proposing a height of 350 feet, which it maintains would not shadow protected Union Square and still be financially viable, despite lopping off more than 100 feet of the most lucrative space.

The developer, Millennium Partners, has agreed to provide more than $20 million toward the roughly $40 million cost of the museum, plus another $5 million toward the museum's operating endowment, Lee said.

Unanimous approval

The envisioned 52,000-square-foot museum in the lower floors of the $260 million tower at 706 Mission St. has been years in the making. It was approved unanimously by the Board of Supervisors.

Schoenberg still likes his chances with a ballot measure banning shadows on public parks.

"We have done our own polling on the ballot measure," he said. "In excess of 60 percent of the people in San Francisco would favor a measure like this."